Digital-to-analog converters (DACs) are well known in the art and are used to decode a digital input signal to a corresponding output analog signal. DACs that are configured to operate in voltage mode, in which an output analog voltage that corresponds to an input digital signal is produced, typically have their outputs buffered by an amplifier (e.g., an operational drive amplifier).
The output range of a DAC is an important consideration for DAC design and implementation. FIG. 1 illustrates example DAC circuits with various DAC range modification solutions. Referring to FIG. 1, there are illustrated DAC circuits 100, 102, and 104. The DAC circuit 100 includes a DAC impedance string 110 and a buffer amplifier 114. The DAC impedance string 110 can be a ladder DAC, such as an R/2R DAC using a plurality of R/2R voltage divider chains. In this regard, the DAC circuit 100 includes R/2R range scaling. An optional gain resistor 112 can be used at the output of the DAC to further scale the DAC 110 output before the amplifier 114.
The DAC circuit 102 includes a DAC impedance string (RDAC) 118, a buffer amplifier 120, and a gain resistor 116. The DAC impedance string 118 can be an R/2R DAC using a plurality of R/2R voltage divider chains. The gain resistance 116 can be equal to the resistance RDAC, and can be coupled in series with the DAC impedance string 118. In this regard, the DAC circuit 102 includes dual string range scaling, where the DAC output range can be scaled within the DAC impedance string 118 and or using the gain resistor 116.
The DAC circuit 104 uses dual string range selection. More specifically, the DAC impedance string 124 is coupled in series to a first gain resistor 122 and a second gain resistor 126. The first gain resistor 122 can be used for scaling down the output range of the DAC impedance string 124, and the second gain resistor 126 can be used to shift up the output range of the DAC impedance string 124.
The DAC circuit termination options illustrated in FIG. 1 have certain drawbacks resulting from the resistors in series connections. More specifically, the DAC circuits illustrated in FIG. 1 can be characterized with increased output impedance, which limits the DAC speed (e.g., settling speed) and increases output noise and glitching.